Recently, liquid crystal display devices have been used in various devices having various sizes, e.g., in a television, a monitor, a mobile phone, and the like. While, a liquid crystal display device raises such a problem that an image blurs in displaying a moving image since its drive mode is a hold drive mode and response of liquid crystal is poor.
As an example of a technique for preventing the blurring image caused by the hold drive mode, black insertion display is known. The black insertion display is such that: a single frame is divided into plural sub frames, and pseudo impulse drive is performed in which at least one sub frame is set as a display period and at least other one sub frame is set as a black display period, so as to suppress the image from blurring.
In the black insertion display, ON/OFF of a backlight is controlled or black is written into a liquid crystal panel so as to carry out black display during a black display period. Recently, also a mobile device such as a mobile phone or the like is required to have a function for displaying a moving image. It is preferable to arrange the mobile device so that black insertion display is carried out by entirely controlling ON/OFF a backlight in order to simplify a device configuration and control means. Note that, it is general that the black insertion display means a method in which black display is inserted by writing black data into a liquid crystal panel. However, in this specification, a display method in which a black display period and an image display period are provided in a simple manner is referred to as “black insertion display”, and also a method in which a black display period and an image display period are provided by controlling ON/OFF of the backlight is referred to as “black insertion display”.
However, the liquid crystal display device has such a problem that response of liquid crystal is poor as described above, so that it is often that a moving image cannot be sufficiently prevented from blurring merely by carrying out the black insertion display. That is, in the black insertion display, writing into pixels is carried out during the black display period, but liquid crystal response caused by the writing has to be completed during the display period. However, writing into upper pixels of a display screen is carried out at the initial time of the black display period, so that a liquid crystal response period can be sufficiently obtained before the display period, but writing into lower pixels of the screen is carried out at the last time of the black display period, so that a period shifting to the display period is short, which results in incomplete liquid crystal response. This may cause the upper portion and the lower portion of the display screen to be different from each other in effect of suppression of blur of the moving image.
Patent Literature 1 discloses a liquid crystal driving method in which the black insertion display is carried out in combination with preliminary writing and overshoot drive so as to suppress blur of the moving image which blur cannot be prevented merely by the black insertion display.
In the liquid crystal driving method of Patent Literature 1, two writing operations, i.e., preliminary writing and regular writing are carried out during the black display period. Thus, it is possible to obtain a liquid crystal response period also in the lower pixels of the display screen by the preliminary writing which is carried out in the first half of the black display period.
Further, Patent Literature 1 discloses an arrangement in which overshoot drive is performed at the time of the preliminary writing so as to further enhance speed of the liquid crystal response. The overshoot drive is the following technique: If a direction in which a current gray scale changes to a gray scale to be displayed is a positive direction, a voltage higher than a writing voltage for the gray scale to be displayed is applied during a predetermined period, and if the direction in which the current gray scale changes to the gray scale to be displayed is a negative direction, a voltage lower than the writing voltage for the gray scale to be displayed is applied during a predetermined period, so as to promote a change of orientation of liquid crystal molecules, thereby enhancing a response property of liquid crystal. That is, in case of changing a first transmittance of a target pixel into a second transmittance higher than the first transmittance, a voltage higher than a writing voltage corresponding to the second transmittance is applied during a predetermined period.